In some high dynamic weak signal scenarios, the carrier phase is difficult to lock. In order to track the high dynamic weak Global Navigation Satellite System(GNSS) signal, considering that the Frequency Locked Loop(FLL) is more robust than the Phase-Locked Loop(PLL), an algorithm based on FLL+differential demodulation is proposed to track and demodulate the GNSS signal. The algorithm uses the second-order FLL to track the frequency of satellite signal, and the differential demodulation algorithm is employed to demodulate bit data. In engineering application, the algorithm adopts the architecture of Field Programmable Gate Array+Digital Signal Processing(FPGA+DSP), realizes the pre-processing of signal tracking signal in FPGA, and realizes the tracking loop algorithm, bit synchronization and differential modulation in DSP. The simulation of the algorithm is performed in Matlab platform, and the algorithm is verified by receiving the real GNSS signal from the simulator platform and the sky. The simulation and experimental results show that the algorithm can realize the stable tracking of satellite signal and data demodulation under the condition of high dynamic and weak signal, and overcome the problem that the data cannot be demodulated due to the difficulty of PLL locking, finally realize the Position Velocity and Time(PVT) solution of GNSS signal under this condition.